Soendenbroe, C.; Andersen, J. L.; Heisterberg, M. F.; Kjaer, M.; Mackey, A. L.

Background: Heavy resistance exercise training (HReT) effectively increases muscle mass and strength in groups of older individuals. However, the extent of inter-individual variability in response to HReT, and the possible existence of non-responders, remains unclear. The primary aim was to determine the presence of inter-individual variability in the response to prolonged HReT in healthy older individuals. Secondary aims were to classify individual responsiveness using a combination of gold-standard assessment methods, and to explore training progression and baseline levels as possible response moderators. Methods: We conducted a secondary analysis of an 8- and 16-week intervention of thrice weekly HReT (EX) or continuation of a sedentary lifestyle (SED). 58 healthy men (age 72+/-5) were randomized into EX (n=38) or SED (n=20). Assessments were conducted at baseline, mid-intervention (8wk), and post-intervention (16wk) for five outcomes: Maximal voluntary contraction strength (MVC), rate of force development (RFD), quadriceps cross-sectional area (qCSA), and type I and II myofibre cross-sectional area (fCSA). Training compliance and progression (as measured by 1-repetition maximum (1RM)) were monitored. Inter-individual variability was assessed using the standard deviation of individual responses (SDIR). Individual change scores relative to a Typical Error (TE) were used to classify individuals as Poor, Trivial, Robust or Excellent responders. Results: At the group level, EX increased MVC, RFD, qCSA and type II fCSA by 19+/-14 %, 58+/-80 %, 3+/-4 % and 14+/-25 %, respectively, with no changes in SED. Inter-individual variability was observed for all outcomes. Individual responses to HReT were outcome- and time-dependent, with 31 participants (82%) being classified as Robust or Excellent responders, and two participants (5%) being Poor responders, following 16wk of HReT. Training compliance and 1RM progression did not account for the observed response variability following HReT. Lower baseline levels were associated with greater improvements; however, this did not account for the observed inter-individual variability. Conclusions: This study provides strong evidence of inter-individual variability in response to HReT among healthy older men. Given the rarity of true non-responders, HReT should remain the universally recommended first-line strategy for enhancing muscle mass and strength in this population.